Although aluminum is ordinarily considered a bright metal, the surface appearance is generally specified by the customer as either having a semi-specular (matte-like) finish or a specular finish. In lighting applications, it is especially desirable that the aluminum have a highly reflective surface, regardless of the specularity of the finish. As used herein, the term "total reflectance" refers to the amount of incident light striking a surface that is reflected in any direction, and the term "highly reflective" refers to a surface which reflects 80% or more. As used herein, the term "specular reflectance" refers to reflectance measured at an angle which is equal to the angle of incidence. The matte-like or semi-specular fmish is defined as an appearance which has a specular reflectance of less than 40%, while the specular finish refers to the finish which has a specular reflectance of greater than 40%, both measured at 30 degrees from normal incident light, per ASTM E-430.
Some known processes for polishing aluminum to produce a highly reflective surface include chemical polishing or electropolishing, both generally carried out in an acidic bath. After polishing, the surface must be treated again to render it resistant to corrosion. In the prior art, corrosion resistance has generally been imparted to aluminum alloy surfaces by anodizing and then coating with a polymer layer. Nikaido et al U.S. Pat. No. 3,945,899 is an example of one prior art reference disclosing anodization of an aluminum alloy surface followed by coating, preferably with an organic polymer such as an acrylic resin or acrylic modified polyester.
Anodizing processes have been practiced commercially on aluminum lighting sheet products for several years. Although anodized surfaces are chemically stable and resistant to corrosion, the processes are expensive. In addition, anodized aluminum alloy surfaces are often subject to some iridescence and to some oxide crazing during subsequent forming or exposure to elevated temperatures.
A principal objective of the present invention is to produce an aluminum sheet having a highly reflective and corrosion-resistant surface, in either a specular or semi-specular finish, without anodizing the surface. The term "corrosion resistant" refers to a product that does not delaminate, peel or significantly yellow or whiten when exposed to 1,000 hours of cycled condensing humidity and UV light, per ASTM G-53.
A related objective of the invention is to provide a process for making aluminum alloy sheet with improved characteristics, such as improved resistance to crazing and improved control of iridescence while maintaining acceptable levels of scratch and dust resistance, formability, appearance, optical performance and long term durability, compared with prior art processes relying upon anodizing.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.